Localised heating and intensive magmatic conditions prior to the 22-23 April 2015 Calbuco volcano eruption (Southern Chile)

Calbuco volcano is a Late-Pleistocene composite stratovolcano and member of the Southern Volcanic Zone of the Chilean Andes (41º19’S, 72º36’W). It lies ~20 km west of the Liquiñe-Ofqui Fault Zone, but is not located directly upon any major regional structures. During April 2015, a sub-Plinian eruption occurred, with a bulk erupted volume of ~0.3-0.6 km3 (~0.1-0.2 km3 DRE). The eruption was a rapid-onset event that produced highly-crystalline products (from 40 to 60 volume %) including the mineral phases: plagioclase, clinopyroxene, orthopyroxene, amphibole, olivine, apatite, ilmenite, titanomagnetite, and chalcopyrite. An upper-crustal reservoir is inferred using available geophysical data combined with amphibole geobarometry. Consideration of textural features, including high crystallinity, complex mineral zonation, crystal clots, and interstitial glass between crystals from clots suggests the presence of a mush zone within this reservoir.

From the nine collected samples, whole-rock chemistry and an array of geothermometers (amphibole, amphibole-plagioclase, two-pyroxenes, and Fe-Ti oxides) gave similar results for all samples possessing ~40 volume % of crystals, with the exception of the sample Cal-160 (~60 volume % crystallinity), which is slightly more evolved and yields lower temperatures for all geothermometers. By comparing temperatures calculated in sample Cal-160 using pairs of ilmenite-titanomagnetite core compositions with those calculated using rim compositions we observe a late-stage temperature increase of between 70 ºC and 200 ºC. We suggest that this local-scale heating event was at least partly responsible for triggering the eruption.

Our data suggest that the bulk of the erupted magma was derived from a relatively uniform (970–1,000ºC), crystal-rich magma mass. Sample Cal-160 was derived from a cooler environment (910–970 ºC), where it was subjected to pre-eruptive heating to temperatures considerably higher than those observed in associated, erupted magmas (up to 1,070 ºC). This requires the involvement of a hot, presumably mafic magma injection at the base of a shallow, crystal-rich reservoir, though the mafic magma was not itself erupted. The localised nature of interaction and rapidity of eruption onset have implications for potential future hazards at Calbuco volcano.